The "power" of an air compressor
The subtext of this title is "how to cut through bulls**t claims about
compressor performance in catalogues". There's a lot of deliberately
misleading claims put out.
The "power" of a compressor is how much air it can pump at a given
pressure in a given time. That is its output. At the bottom line,
what a compressor does is supply atmospheric air but at a raised
pressure. You cram more air into a given volume than is the there is
at atmospheric pressure and supply this to whatever is demanding it.
The "Free Air Delivery" (FAD)
The above is known as the "Free Air Delivery" (FAD). There would be a
problem quoting this if you did not standardise the pressure. Then
even if you did, you would not be able to compare the fall-off in
pumping capacity of a particular compressor with increasing output
pressure. So this is what FAD is about:
You quote the FAD at a particular output pressure. That would
normally be 90 p.s.i, which most air tools work at.
Imagine this - with the compressor running and supplying air at the
The volume of the air which is now back at atmospheric pressure is the
Free Air Delivery. Divide that volume by the time it took for the
compressor to pump it and you have the rate of FAD.
- you start a clock and pump air for a given period of time, drawing
off the air at the stated pressure as fast as keeps up with the
compressor's pumping of new air
- you take this pressurised air which has been pumped in the length
of time you measured
- you expand it back to atmospheric pressure, but collecting it
- you measure its volume
The traditional unit of the rate of FAD in Britain and America is
Cubic Feet of air per Minute (c.f.m.).
You can expect certain FAD rates from certain classes of compressor.
In the UK the most powerful single-phase motor you can connect to the
domestic mains is 3HP (3 horse-power). A portable 3HP compressor will
pump about 7 c.f.m. at 90 p.s.i.. An
stationary good quality garage compressor will deliver about
10.5 c.f.m. at 90 p.s.i.. With a two-stage
compressor with intercooling between the stages you can get a 25%
higher FAD for the same motor power - an option which will cost you
about 40% more but is for real.
How you "cheat" to give higher output figures
In at number one is quoting the displacement of the compressor, not
the air it delivers. If you multiply the volume of the cylinder(s) by
the number of times they reciprocate in one minute (= the
crankshaft rotational speed), you will get the displacement. This is
higher than the actual air delivered. This is because some of the air
is not pushed into the tank of the compressor because of the
back-pressure of air in the tank. Taking my compressor: its FAD is 7 c.f.m. at 90 p.s.i., whereas its swept volume is 11 c.f.m.. That is the typical ratio of FAD to swept volume for a single-stage compressor.
In at number two is quoting the FAD but at a lower pressure like
30 p.s.i.. That figure will be low double figures of
percent higher than the FAD at 90 p.s.i., for a good
compressor. If the compressor is not very good and struggles at a
proper working pressure like 90 p.s.i. then the
difference could be greater; a reason for quoting at this lower
Outright creating a fiction is another possibility
Even with true FAD, not all things are equal
Knowing that FAD at 90 p.s.i. is the standard for rating
a compressor, you can create a very cheap machine which will sound
just fine compared to the performance of a machine from a reputable
manufacturer. But it won't last long. And here is how you do it.
You take a cheaper smaller inadequately powerful motor and throw more
load on it than it is supposed to take. Then you couple it to a
smaller cheaper compressor (pump) and spin it at the maximum it will
take in order to get a lot of air out of it. The machine is not going
to last very long. You are paying a bit less for a lot less than if
you got a proper well-specified machine.
(Richard Smith, June 2005)